1. Field of the Invention
The present invention relates to optical transmission systems, more specifically to a system for optically transmitting an angle-modulated signal.
2. Description of the Background Art
FIG. 30 is a block diagram showing an example of the configuration of a conventional optical transmission system which transmits an angle-modulated signal. In FIG. 30, the optical transmission system includes an angle modulating portion 1, an optical modulating portion 2, an optical waveguide portion 3, an optical/electrical converting portion 4, an angle demodulating portion 5 and a filter F. Such optical transmission system is described, for example, in a document (K. Kikushima, et al., xe2x80x9cOptical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systemsxe2x80x9d, IOOC""95, PD2-7, 1995, pp. 33-34.).
Next, the operation of the conventional optical transmission system structured as above will be described. As an electrical signal inputted to the angle modulating portion 1, assumed is an analog signal such as an audio or video signal, or a digital signal such as computer data and the like. The angle modulating portion 1 converts the inputted electrical signal into an angle-modulated signal with a predetermined frequency and a predetermined angle modulation scheme to output the angle-modulated signal. The angle modulation scheme includes FM (frequency modulation) or PM (phase modulation) for an analog signal and FSK (frequency-shift keying) or PSK (phase-shift keying) for a digital signal, and is generically referred to as angle modulation hereinafter. The optical modulating portion 2 converts the inputted angle-modulated signal into an optical-modulated signal to output the optical-modulated signal. The optical/electrical converting portion 4, which includes a photodetector having square-law-detection characteristics (a pin photo-diode, an avalanche photo-diode or the like), re-converts the optical-modulated signal transmitted by the optical waveguide portion 3 into an electrical signal to output an angle-modulated signal. The angle demodulating portion 5 converts variations in frequency (or variations in phase) of the angle-modulated signal into variations in amplitude (or variations in intensity) of an electrical signal, thereby re-generating a signal correlating with the original electrical signal. The filter F passes only a signal component corresponding to the original electrical signal (that is a signal component of the same frequency band as that of the original electrical signal) among signals outputted from the angle demodulating portion 5.
In FIG. 31 is shown an example of the structure of the angle demodulating portion 5 in FIG. 30. In FIG. 31, the angle-modulated signal inputted from the optical/electrical converting portion 4 is branched into two signals in a branch portion 51. One signal of the two signals obtained by the branch is provided with a predetermined delay Tp in a delay portion 52. A mixing portion 53, which is generally constituted by a mixer and the like, receives the other signal outputted from the branch portion 51 and the signal outputted from the delay portion 52 to generate a product signal of these signals and output the product signal.
The conventional optical transmission system of the angle-modulated signal as described above has an advantage in the following, compared with an optical transmission system of an amplitude-modulated (AM) signal. That is, the frequency deviation (or the phase deviation) of the angle-modulated signal is set larger, so that a larger gain in angle modulation can be acquired at the optical transmission. As a result, SNR (signal-to-noise power ratio) of a demodulated signal increases, realizing transmission of a signal of good quality. Moreover, the frequency deviation (or the phase deviation) of the angle-modulated signal is increased to spread a frequency spectrum of the optical-modulated signal and suppress a peak level of the frequency spectrum, which leads to an advantage in that deterioration of signal quality due to multipath reflection on an optical transmission line is reduced.
As described above, in the conventional optical transmission system, an electrical signal to be transmitted, after being subjected to angle modulation, is converted into an optical-modulated signal to be optically transmitted, subjected to square-law-detection on a receiving side to be re-converted into an angle-modulated signal, and further subjected to angle demodulation to be the original electrical signal. Therefore, it is possible, in the conventional optical transmission system, to perform optical transmission of better quality by increasing the frequency deviation (the phase deviation) even on an optical transmission line of poor quality.
However, increasing in the frequency deviation (or the phase deviation) of the angle-modulated signal makes the frequency and band of the angle-modulated signal higher and wider. Accordingly, the conventional optical transmission system as described above, requires electrical parts for high frequencies and wide-bands in order to constitute the angle modulating portion 1 and the angle demodulating portion 5. Connection and matching among such electrical parts for high frequencies and wide-bands are difficult and multipath reflection among the parts readily occurs. This causes deterioration of characteristics of the angle modulating portion 1 and the angle demodulating portion 5, resulting in significant deterioration of quality of modulated/demodulated signals.
Further, in the case where an expensive electrical part for wide-bands and high frequencies (for example, the branch portion 51 and the mixing portion 53 in FIG. 31) is used in the angle demodulating portion 5 which is installed as a receiving terminal of an optical transmission system, when configuring an optical subscriber (optical multi-distribution) system such as a FTTH (Fiber To The Home) system, a CATV network and the like, the system cost per subscriber becomes very high to significantly degrade the system from the view point of its economy.
As explained in the above, the conventional optical transmission system is required, when optically transmitting an angle-modulated signal with a wider-band and a higher frequency, to use the electrical parts for wide-bands and high frequencies especially as constituents of the demodulating portion. Thereby, the conventional optical transmission system has a specific problem in that group delay characteristics and modulation/demodulation characteristics are easily deteriorated and economy of overall system is significantly degraded because of increase in the cost of the receiving terminal.
Therefore, an object of the present invention is to provide an optical transmission system which realizes good angle demodulation characteristics by adopting new optical signal processing and is greatly economical by constituting a receiving terminal at lower cost without electrical parts for wide-bands and high-frequencies.
The present invention has features described below in order to attain the above-mentioned object.
A first aspect of the present invention is an optical transmission system for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal into an optical-modulated signal;
an interference portion for separating the optical-modulated signal into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal,
the interference portion and the optical/electrical converting portion constituting a delayed detection system of an optical signal, and the delayed detection system performing conversion processing of an optical signal into an electrical signal and angle demodulation processing simultaneously.
In the case where an electrical circuit using parts for wide-bands and high-frequencies is adopted as a demodulation device for an angle-modulated signal, connection or matching among the parts are difficult, causing deterioration of linearities of demodulation characteristics or group delay characteristics easily to degrade the quality of an demodulated signal. Moreover, the parts for wide-bands and high-frequencies are generally expensive, so that the cost of the demodulation device increases, significantly deteriorating the economy of the system.
Hence, in the above first aspect, an angle-modulated signal is converted into an optical-modulated signal and the optical-modulated signal is homodyne detected employing square-law-detection characteristics of a photodetector, so that demodulation and optical transmission can be performed only by optical signal processing without using electrical parts for wide-bands and high-frequencies. Further, when the present aspect is applied to an optical distribution system, the portions in the configuration up to the interference portion are installed on a transmitting equipment side and only the optical/electrical converting portion is installed on a receiving terminal side, whereby the expensive constituents are included in only the transmitting equipment. Thus, it is possible to construct an optical subscriber system which is greatly economical.
A second aspect is an aspect according to the first aspect, wherein the angle-modulated signal is an FM signal obtained by subjecting an analog signal to frequency modulation.
A third aspect is an aspect according to the first aspect, wherein the angle-modulated signal is a PM signal obtained by subjecting an analog signal to phase modulation.
A fourth aspect is an aspect according to the first aspect, wherein the angle-modulated signal is an FSK modulated signal obtained by subjecting a digital signal to frequency modulation.
A fifth aspect is an aspect according to the first aspect, wherein the angle-modulated signal is a PSK modulated signal obtained by subjecting a digital signal to phase modulation.
A sixth aspect is an aspect according to claim 1, wherein the optical modulating portion generates an optical-intensity-modulated signal as the optical-modulated signal.
A seventh aspect is an aspect according to the sixth aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a given wavelength;
an optical branch portion for branching the light from the light source into two;
first and second optical phase modulating portions, provided for the two outputted lights from the optical branch portion respectively, for subjecting the outputted lights to optical phase modulation using the angle-modulated signal as an original signal; and
an optical coupling portion for combining the two optical-phase-modulated signals outputted from the first and second optical phase modulating portions.
As described in the foregoing, in the seventh aspect, in order to generate an optical-intensity-modulated signal, an external modulation scheme is adopted. In place of such external modulation scheme, a direct modulation scheme can be also adopted.
An eighth aspect is an aspect according to the sixth aspect, wherein
the interference portion comprises:
an optical branch portion for branching an inputted optical signal into a first optical signal and a second optical signal;
an optical delay portion for providing the second optical signal outputted from the optical branch portion with a predetermined delay; and
an optical combining portion for combining the first optical signal outputted from the optical branch portion and the second optical signal outputted from the optical delay portion.
As described in the foregoing, in the eighth aspect, the inputted optical signal is branched into two optical signals by the optical branch portion, the predetermined propagation delay is provided for one of the two optical signals and then the two optical signals are combined again by the optical combining portion, which constitutes an interference system necessary for delayed detection of an optical signal.
A ninth aspect is an aspect according to the sixth aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a given wavelength;
an optical branch portion for branching the light from the light source into two;
first and second optical phase modulating portions, provided for the two outputted lights from the optical branch portion respectively, for each subjecting each of the outputted lights to optical phase modulation using the angle-modulated signal as an original signal; and
an optical directional coupling portion for combining the two optical-phase-modulated signals outputted from the first and second optical phase modulating portions and then dividing the resultant signal into first and second optical signals in which optical-intensity modulated components are set in opposite phases to each other, and
the interference portion comprises:
an optical delay portion for providing the second optical signal outputted from the optical directional coupling portion with a predetermined delay; and
an optical combining portion for combining the first optical signal outputted from the optical directional coupling portion and the second optical signal outputted from the optical delay portion.
As described in the foregoing, in the ninth aspect, the external modulation scheme is adopted in the optical modulating portion and the optical directional coupling portion is provided, to input the first and second optical signals, in which optical-intensity-modulated components are set in opposite phases to each other, to the interference portion. This eliminates the need for branching the inputted optical signal in the interference portion.
A tenth aspect is an aspect according to the sixth aspect, wherein
the interference portion comprises:
an optical waveguide portion for guiding the optical signal outputted from the optical modulating portion; and
first and second optical transparent/reflecting portions, cascaded on the optical waveguide portion at a prescribed interval, for respectively transmitting parts of the inputted optical signals and reflecting the remained parts, and
propagation time in which an optical signal goes and returns between the first and second optical transparent/reflecting portions is the predetermined difference in propagation delay.
As described in the foregoing, according to the tenth aspect, two optical transparent/reflecting portions are provided on the optical waveguide portion, and a direct light which propagates through both of the optical transparent/reflecting portions and an indirect light which goes and returns between the optical transparent/reflecting portions one time and then propagates are generated, which constitutes an interference system necessary for delayed detection of an optical signal without physically branching the optical signal into two. This allows constitution of the interference system with a simpler configuration.
An eleventh aspect is an aspect according to the first aspect, wherein the optical modulating portion generates an optical-amplitude-modulated signal as the optical-modulated signal.
A twelfth aspect is an aspect according to the eleventh aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a given wavelength;
an optical branch portion for branching the light from the light source into two;
first and second optical phase modulating portions, provided for the two outputted lights from the optical branch portion respectively, for each subjecting each of the outputted lights to optical phase modulation using the angle-modulated signals as an original signal; and
an optical coupling portion for combining the two optical-phase-modulated signals outputted from the first and second optical phase modulating portions.
A thirteenth aspect is an aspect according to the eleventh aspect, wherein
the interference portion comprises:
an optical branch portion for branching the inputted optical signal into a first optical signal and a second optical signal;
an optical delay portion for providing the second optical signal outputted from the optical branch portion with a predetermined delay; and
an optical combining portion for combining the first optical signal outputted from the second optical branch portion and the second optical signal outputted from the optical delay portion.
As described in the foregoing, according to the thirteenth aspect, the inputted optical signal is branched into two optical signals by the optical branch portion, the predetermined propagation delay is provided for one of the two optical signals in the delay portion and then the two optical signals are combined again in the optical combining portion, to constitute an interference system necessary for delayed detection of an optical signal.
A fourteenth aspect is an aspect according to the eleventh aspect, wherein
the optical modulating portion comprises:
a light source for outputting a light with a given optical intensity and a given wavelength;
an optical branch portion for branching the light from the light source into two;
first and second optical phase modulating portions, provided for the two outputted lights from the optical branch portion respectively, for each subjecting each of the outputted lights to optical phase modulation using the angle-modulated signal as an original signal; and
an optical directional coupling portion for combining the two optical-phase-modulated signals outputted from the first and second optical phase modulating portions and then dividing the resultant signal into first and second optical signals in which optical-amplitude-modulated components are set in opposite phases to each other, and
the interference portion comprises:
an optical delay portion for providing the second optical signal outputted from the optical directional coupling portion with a predetermined delay; and
an optical combining portion for combining the first optical signal outputted from the optical directional coupling portion and the second optical signal outputted from the optical delay portion.
As described in the foregoing, in the fourteenth aspect, the external modulation scheme is adopted in the optical modulating portion and the optical directional coupling portion is provided, to input the first and second optical signals, in which optical-intensity-modulated components are set in opposite phases to each other, to the interference portion. This eliminates the need for branching the inputted optical signal in the interference portion.
A fifteenth aspect is an aspect according to the eleventh aspect, wherein
the interference portion comprises:
an optical waveguide portion for guiding the optical signal outputted from the optical modulating portion; and
first and second optical transparent/reflecting portions, cascaded on the optical waveguide portion at a predetermined interval, for respectively transmitting parts of the inputted optical signals and reflecting the remained parts, and
propagation time in which an optical signal goes and returns between the first and second optical transparent/reflecting portions is the predetermined difference in propagation delay.
As described in the foregoing, according to the fifteenth aspect, two optical transparent/reflecting portions are provided on the optical waveguide portion, and the direct light which propagates through both of the optical transparent/reflecting portions and the indirect light which goes and returns between the optical transparent/reflecting portions one time and then propagates are generated, which constitutes an interference system necessary for delayed detection of an optical signal without physically branching the optical signal into two. This allows constitution of the interference system with a simpler configuration.
A sixteenth aspect is an aspect according to the twelfth aspect, wherein predetermined optical phase modulation is performed in the first and second optical phase modulating portions so that difference between the optical phase shift by the first optical phase modulating portion and the optical phase shift by the second optical phase modulating portion is set in phase with the angle-modulated signal.
A seventeenth aspect is an aspect according to the fourteenth aspect, wherein predetermined optical phase modulation is performed in the first and second optical phase modulating portions so that difference between the optical phase shift by the first optical phase modulating portion and the optical phase shift by the second optical phase modulating portion is set in phase with the angle-modulated signal.
An eighteenth aspect is an aspect according to the twelfth aspect, wherein predetermined optical phase modulation is performed in the first and second optical phase modulating portions so that difference between the optical phase shift by the first optical phase modulating portion and the optical phase shift by the second optical phase modulating portion is set in opposite phases with the angle-modulated signal.
A nineteenth aspect is an aspect according to the fourteenth aspect, wherein predetermined optical phase modulation is performed in the first and second optical phase modulating portions so that difference between the optical phase shift by the first optical phase modulating portion and the optical phase shift by the second optical phase modulating portion is set in opposite phases with the angle-modulated signal.
In the sixteenth to nineteenth aspects, phase relation between the angle-modulated signals inputted into the first and second optical phase modulating portions is optimally adjusted, to enlarge the optical-amplitude-modulated component in the optical signal inputted into the optical coupling portion or the optical directional coupling portion, which realizes high efficient demodulation and optical transmission with optical signal processing.
A twentieth aspect is an aspect according to the first aspect, wherein a product value of a center angular frequency of the angle-modulated signal and the predetermined difference in propagation delay in the interference portion is set to be equal to xcfx80/2.
As described in the foregoing, in the twentieth aspect, the center angular frequency of the angle-modulated signal and the predetermined difference in propagation delay in the interference portion are set at optimal values, to increase demodulation efficiency.
A twenty-first aspect is an aspect according to the fourth aspect, wherein the predetermined difference in propagation delay in the interference portion is set to be equal to one symbol length of the digital signal.
A twenty-second aspect is an aspect according to the fifth aspect, wherein the predetermined difference in propagation delay in the interference portion is set to be equal to one symbol length of the digital signal.
As described in the foregoing, in the twenty-first and twenty-second aspects, when the angle-modulated signal is an FSK modulated signal or a PSK modulated signal obtained by subjecting a digital signal to frequency modulation or phase modulation, the symbol length of the digital signal and the predetermined difference in propagation delay in the interference portion are set to optimal values, thereby increasing the demodulation efficiency.
A twenty-third aspect is an aspect according to the eighth aspect, wherein polarization states of the first optical signal and the second optical signal to be combined in the optical combining portion are set to be the same with each other.
A twenty-fourth aspect is an aspect according to the ninth aspect, wherein polarization states of the first optical signal and the second optical signal to be combined in the optical combining portion are set to be the same with each other.
A twenty-fifth aspect is an aspect according to the thirteenth aspect, wherein polarization states of the first optical signal and the second optical signal to be combined in the optical combining portion are set to be the same with each other.
A twenty-sixth aspect is an aspect according to the fourteenth aspect, wherein
polarization states of the first optical signal and the second optical signal to be combined in the optical combining portion are set to be the same with each other.
As described in the foregoing, in the twenty-third to twenty-sixth aspects, the polarization states of the first and second optical signals in the optical combining portion are set to be the same with each other, thereby increasing homodyne detection efficiency in the optical/electrical converting portion, that is demodulation efficiency.
A twenty-seventh aspect is an aspect according to the tenth aspect, wherein
polarization states of the optical signal transmitting through the first and second optical transparent/reflecting portions along the optical waveguide portion and the optical signal transmitting through the first optical transparent/reflecting portion, reflected at the second optical transparent/reflecting portion, reflected at the first optical transparent/reflecting portion and transmitting through the second optical transparent/reflecting portion are set to be the same with each other.
A twenty-eighth aspect is an aspect according to the fifteenth aspect, wherein
polarization states of the optical signal transmitting through the first and second optical transparent/reflecting portions along the optical waveguide portion and the optical signal transmitting through the first optical transparent/reflecting portion, reflected at the second optical transparent/reflecting portion, reflected at the first optical transparent/reflecting portion and transmitting through the second optical transparent/reflecting portion are set to be the same with each other.
As described in the foregoing, in the twenty-seventh and twenty-eighth aspects, the polarization states of the direct light and the indirect light are set to be the same, thereby increasing the homodyne detection efficiency in the optical/electrical converting portion, that is the demodulation efficiency.
A twenty-ninth aspect is an aspect according to the eighth aspect, wherein
the optical modulating portion and the interference portion are connected with a first optical waveguide portion,
the interference portion and the optical/electrical converting portion are connected with a second optical waveguide portion, and
the first and/or second optical waveguide portions are composed of single-mode optical fibers.
The thirtieth aspect is an aspect according to the thirteenth aspect, wherein
the optical modulating portion and the interference portion are connected with a first optical waveguide portion,
the interference portion and the optical/electrical converting portion are connected with a second optical waveguide portion, and
the first and/or second optical waveguide portions are composed of single-mode optical fibers.
As described in the foregoing, in the twenty-ninth and thirtieth aspects, the first and/or second optical waveguide portions are composed of single-mode optical fibers, making it possible to perform optical transmission with optical fibers which are inexpensive.
A thirty-first aspect is an aspect according to the ninth aspect, wherein
the interference portion and the optical/electrical converting portion are connected with an optical waveguide portion, and
the optical waveguide portion is composed of a single-mode optical fiber.
A thirty-second aspect is an aspect according to the fourteenth aspect, wherein
the interference portion and the optical/electrical converting portion are connected with an optical waveguide portion, and
the optical waveguide portion is composed of a single-mode optical fiber.
As described in the foregoing, in the thirty-first and thirty-second aspects, the optical waveguide portion provided between the interference portion and the optical/electrical converting portion is composed of a single-mode optical fiber, making it possible to perform optical transmission with an optical fiber which is inexpensive.
A thirty-third aspect is an aspect according to the tenth aspect, wherein a whole or a part of the optical waveguide portion in the interference portion is composed of a single-mode optical fiber.
A thirty-fourth aspect is an aspect according to the fifteenth aspect, wherein a whole or a part of the optical waveguide portion in the interference portion is composed of a single-mode optical fiber.
As described in the foregoing, in the thirty-third and thirty-fourth aspects, the whole or a part of the optical waveguide portion in the interference portion is composed of a single-mode optical fiber, allowing optical transmission with an optical fiber which is inexpensive.
A thirty-fifth aspect is an aspect according to the first aspect, further comprising an amplitude adjusting portion for adjusting an amplitude of the angle-modulated signal and outputting the angle-modulated signal of a constant amplitude.
In the case where delayed detection is performed employing the square-law-detection characteristics of the optical/electrical converting portion, as the amplitude of the angle-modulated signal which is the original signal becomes smaller, the demodulation efficiency decreases. Further, when the angle-modulated signal has an amplitude fluctuation, deterioration in signal quality such as waveform distortion and the like occurs. Hence, in the above thirty-fifth aspect, the amplitude adjusting portion maintaining the amplitude constant is provided for the inputted angle-modulated signal, to suppress the above-mentioned deterioration.
A thirty-sixth aspect is an aspect according to the first aspect, further comprising a bandwidth limiting portion for limiting a band of the angle-modulated signal.
As described in the foregoing, in the thirty-sixth aspect, the bands of the angle-modulated signal is previously limited, to lessen the spectrum in width, thereby preventing deterioration in quality of a demodulated signal caused by that the part of the spread spectrum of the angle-modulated signal component is superimposed on the band of the demodulated signal outputted from the optical/electrical converting portion.
A thirty-seventh aspect is an optical transmitter for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal into an optical-modulated signal; and
an interference portion for separating the optical-modulated signal into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals, and
the optical transmitter transmitting the combined optical signal outputted from the interference portion.
A thirty-eighth aspect is an aspect according to the thirty-seventh aspect, wherein the angle-modulated signal is an FM signal obtained by subjecting an analog signal to frequency modulation.
A thirty-ninth aspect is an aspect according to the thirty-seventh aspect, wherein the angle-modulated signal is a PM signal obtained by subjecting an analog signal to phase modulation.
A fortieth aspect is an aspect according to the thirty-seventh aspect, wherein the angle-modulated signal is an FSK modulated signal obtained by subjecting a digital signal to frequency modulation.
A forty-first aspect is an aspect according to the thirty-seventh aspect, wherein the angle-modulated signal is a PSK modulated signal obtained by subjecting a digital signal to phase modulation.
A forty-second aspect is an aspect according to the thirty-seventh aspect, wherein the optical modulating portion generates an optical-intensity- modulated signal as the optical-modulated signal.
A forty-third aspect is an aspect according to the thirty-seventh aspect, wherein the optical modulating portion generates an optical-amplitude- modulated signal as the optical-modulated signal.
A forty-fourth aspect is an optical receiver for receiving an optical-modulated signal and acquiring a demodulated signal of the optical-modulated signal, comprising:
an interference portion for separating the received optical-modulated signal into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal, and
the interference portion and the optical/electrical converting portion constituting a delayed detection system of an optical signal and the delayed detection system performing conversion processing of an optical signal into an electrical signal and angle demodulation processing simultaneously.
A forty-fifth aspect is an aspect according to the forty-fourth aspect, wherein
the optical-modulated signal is generated from a 2n-phase (n is an integer of not less than two) PSK electrical-modulated signal as an original signal,
the interference portion includes:
a received light dividing portion for dividing an inputted optical signal into 2nxe2x88x921 received lights; and
first to 2nxe2x88x921th optical interference circuits, provided corresponding to the 2nxe2x88x921 received lights respectively, for each branching each of the received lights into a first optical signal and a second optical signal, providing the second optical signal with a predetermined delay and then combining the first and second optical signals, and
the optical/electrical signals are provided corresponding to the first to 2nxe2x88x921th optical interference circuits respectively.
A forty-sixth aspect is an aspect according to the forty-fifth aspect, wherein
the optical-modulated signal is generated from a quadrature PSK electrical-modulated signal as an original signal,
the interference portion includes:
a received light dividing portion for dividing an inputted optical signal into a first received light and a second received light;
a first optical interference circuit for branching the first received light into a first optical signal and a second optical signal, providing the second optical signal with a first predetermined delay and then combining the first and second optical signals; and
a second optical interference circuit for branching the second received light into a first optical signal and a second optical signal, providing the second optical signal with a second predetermined delay and then combining the first and second optical signals, and
the first predetermined delay in the first optical interference circuit and the second predetermined delay in the second optical interference circuit are both set to have the absolute magnitude of xc2xd symbol length of the digital signal and be in opposite phases to each other.
A forty-seventh aspect is an optical transmission system for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal outputted from the optical modulating portion into two signals at least, a first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal outputted from the optical branch portion into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals;
a first optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal; and
a second optical/electrical converting portion, having square-law-detection characteristics, for converting the second optical-modulated signal outputted from the optical branch portion into an electrical signal.
As described in the foregoing, according to the forty-seventh aspect, an angle-modulated signal is converted into an optical signal and branched into a plurality of optical signals, a part of the optical signals are subjected to homodyne detection by the interference portion and the first optical/electrical converting portion to reproduce the original electrical signal for the angle modulation as described in the first aspect and the remained part of the optical signals are subjected to direct detection by the second optical/electrical converting portion to reproduce the angle-modulated signal. Thereby, if a wired network is constructed by using an optical fiber as its backbone and the angle-modulated signal outputted from the second optical/electrical converting portion is sent out in the air as a radio wave, the optical transmission system can expand to a wireless network for mobile terminals and the like. Especially, a high-frequency signal such as a micro wave, a millimetre wave and the like, which is thought as an suitable signal for a wireless network, is received and subjected to demodulation, in a wired system, by a low cost configuration with optical signal processing and at the same time a radio wave is sent to the mobile terminals and the like, so that a flexible and greatly economical system can be constructed.
A forty-eighth aspect is an aspect according to the forty-seventh aspect, further comprising:
a local light source for outputting a light of a predetermined wavelength; and
an optical combining portion, inserted between the optical branch portion and the second optical/electrical converting portion, for combining the second optical-modulated signal outputted from the optical branch portion and the light from the local light source,
wherein the second optical/electrical converting portion heterodyne detects the combined optical signal outputted from the optical combining portion and then converts the optical signal into an electrical signal.
A forty-ninth aspect is an aspect according to the forty-seventh aspect, further comprising:
a local light source for outputting a light of a predetermined wavelength; and
an optical combining portion, inserted between the optical modulating portion and the optical branch portion, for combining the optical-modulated signal outputted from the optical modulating portion and the light from the local light source,
wherein the second optical/electrical converting portion heterodyne detects the second optical-modulated signal outputted from the optical branch portion and converts the optical-modulated signal into an electrical signal.
As described in the foregoing, according to the forty-eighth and forty-ninth aspects, the frequency of the local light source is varied, to freely up-convert or down-convert the frequency of the angle-modulated signal outputted from the second optical/electrical converting portion.
A fiftieth aspect is an optical transmission system for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal into an optical-modulated signal;
a local light source for outputting a light of a predetermined wavelength;
an optical combining portion for combining the optical-modulated signal outputted from the optical modulating portion and the light from the local light source;
an interference portion for separating the combined optical signal outputted from the optical combining portion into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals;
an optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal; and
a dividing portion for separating the electrical signal outputted from the optical/electrical converting portion for each of frequency components and outputting the electrical signals.
A fifty-first aspect is an optical transmission system for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal outputted from the optical modulating portion into two signals at least, a first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal outputted from the optical branch portion into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals;
a first optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal;
a local oscillation portion for outputting an unmodulated signal of a predetermined frequency; and
a second optical/electrical converting portion, having square-law-detection characteristics, in which its bias is modulated with the unmodulated signal from the local oscillation portion, for converting the second optical-modulated signal outputted from the optical branch portion into an electrical signal.
A fifty-second aspect is an optical transmission system for optically transmitting an angle-modulated signal, comprising:
an optical modulating portion for converting the angle-modulated signal into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal outputted from the optical modulating portion into two signals at least, a first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal outputted from the optical branch portion into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals;
a first optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal;
a second optical/electrical converting portion, having square-law-detection characteristics, for converting the second optical-modulated signal outputted from the optical branch portion into an electrical signal;
a local oscillation portion for outputting an unmodulated signal of a predetermined frequency; and
a mixing portion for mixing the electrical signal outputted from the second optical/electrical converting portion and the unmodulated signal outputted from the local oscillation portion and outputting the resultant signals.
As described in the foregoing, according to the fiftieth to fifty-second aspects, the original electrical signal and the angle-modulated signal for angle modulation can be reproduced only by optical signal processing. Further, the frequency of the local light source or the local oscillation portion is varied, to freely up-convert or down-convert the frequency of the angle-modulated signal to be reproduced.
A fifty-third aspect is an optical transmission system for optically transmitting two signals at least, a first electrical signal and a second electrical signal simultaneously, comprising:
an angle modulating portion for converting the first electrical signal into an angle-modulated signal;
a combining portion for combining the angle-modulated signal and the second electrical signal;
an optical modulating portion for converting the combined signal outputted from the combining portion into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal outputted from the optical modulating portion into two signals at least, a first optical-modulated signal and a second optical-modulated signal;
an interference portion for separating the first optical-modulated signal outputted from the optical branch portion into a plurality of optical signals having predetermined difference in propagation delay and then combining the optical signals;
a first optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal; and
a second optical/electrical converting portion, having square-law-detection characteristics, for converting the second optical-modulated signal outputted from the optical branch portion into an electrical signal.
As described in the foregoing, according to the fifty-third aspect, a digital signal and an analog signal, for example, which are different types of electrical signals, can be optically transmitted simultaneously and individually reproduced.
A fifty-fourth aspect is an aspect according to the fifty-third aspect, wherein an occupied frequency band of the first electrical signal, an occupied frequency band of the second electrical signal and an occupied frequency band of the angle-modulated signal do not overlap with each other.
A fifty-fifth aspect is an aspect according to the fifty-third aspect, further comprising:
a first signal processing portion for limiting the occupied frequency band of the first electrical signal; and
a second signal processing portion for limiting the occupied frequency band of the second electrical signal.
A fifty-sixth aspect is an aspect according to the fifty-fifth aspect, further comprising:
a third signal processing portion for passing only a frequency component corresponding to the occupied frequency band of the first electrical signal as to the electrical signal outputted from the first optical/electrical converting portion and reproducing waveform information which was lost by the band limitation in the first signal processing portion; and
a fourth signal processing portion for passing only a frequency component corresponding to the occupied frequency band of the second electrical signal as to the electrical signal outputted from the second optical/electrical converting portion and reproducing waveform information which was lost by the band limitation in the second signal processing portion.
As described in the foregoing, according to the fifty-sixth aspect, the waveform distortion caused by the band limitation performed on the transmitting side can be corrected on the receiving side.
A fifty-seventh aspect is an optical transmission system for optically transmitting a plurality of electrical signals, comprising:
a plurality of angle modulating portions for converting each of the plurality of electrical signals into an angle-modulated signals;
a combining portion for combining the angle-modulated signals outputted from the plurality of angle modulating portions;
an optical modulating portion for converting the combined signal outputted from the combining portion into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal outputted from the optical modulating portion into a plurality of optical-modulated signals; and
an plurality of optical signal processing portions, provided corresponding to the plurality of optical-modulated signals outputted from the optical branch portion respectively, for each performing predetermined optical signal processing and then individually reproducing the plurality of electrical signals, and
each of the optical signal processing portions including:
an interference portion for separating the optical-modulated signal outputted from the optical branch portion into a plurality of optical signals having difference in propagation delay decided according to frequencies of angle-modulated signals to be acquired by demodulation and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal.
As described in the above, according to the fifty-seventh aspect, a digital signal and an analog signal, for example, which are different types of electrical signals, can be optically transmitted simultaneously and individually reproduced.
A fifty-eighth aspect is an aspect according to the fifty-seventh aspect, wherein occupied frequency bands of the plurality of electrical signals and occupied frequency bands of the plurality of angle-modulated signals do not overlap with each other.
A fifty-ninth aspect is an aspect according to the fifty-seventh aspect, further comprising a plurality of signal pre-processing portions for limiting the occupied frequency bands of the plurality of electrical signals.
A sixtieth aspect is an aspect according to the fifty-ninth aspect, wherein each of the plurality of optical signal processing portions further includes a signal post-processing portion for passing a frequency component corresponding to an occupied frequency band of an electrical signal to be reproduced and reproducing waveform information which was lost by the band limitation in the signal pre-processing portion as to the electrical signal outputted from the optical/electrical converting portion.
As described in the foregoing, according to the sixtieth aspect, the waveform distortion caused by the band limitation performed on the transmitting side can be corrected on the receiving side.
A sixty-first aspect is an optical transmission system for optically transmitting a multichannel angle-modulated signal obtained by subjecting plurality-channel electrical signals to angle modulation respectively and frequency-division multiplexing, comprising:
an optical modulating portion for converting the multichannel angle-modulated signal into an optical-modulated signal;
an optical branch portion for branching the optical-modulated signal outputted from the optical modulating portion into a plurality of optical-modulated signals; and
a plurality of optical signal processing portions, provided corresponding to the plurality of optical-modulated signals outputted from the optical branch portion respectively, for each performing predetermined optical signal processing and then reproducing an electrical signal on an individual channel, and
each of the optical signal processing portions including:
an interference portion for separating the optical-modulated signal outputted from the optical branch portion into a plurality of optical signals having difference in propagation delay decided according to frequencies of electrical signals on channels to be reproduced and then combining the optical signals; and
an optical/electrical converting portion, having square-law-detection characteristics, for converting the combined optical signal outputted from the interference portion into an electrical signal.
As described in the foregoing, according to the sixty-first aspect, the multichannel angle-modulated signal obtained by frequency-division-multiplexing can be optically transmitted simultaneously.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.